Customs officials just stopped the biggest cattle heist to occur in Japan in at least a decade.

It was the biggest little cattle caper to hit Japan in years. A man was recently caught by Chinese customs officers in Shanghai carrying a suspicious container filled with hundreds of fertilized eggs illegally taken from expensive purebred wagyu beef cows, according to reports by the Japanese wire agency Jiji Press.

Japanese Agriculture Minister Takamori Yoshikawa said his office was "taking procedures to file a criminal complaint," against the unidentified man, explaining that the country has strict laws forbidding the export of live wagyu cattle, as well as their sperm and eggs, in order to protect the $170 million USD domestic industry.

The ministry believes this is the first case of wagyu cattle ovum smuggling to occur in Japan in at least a decade—although Yoshikawa admitted that the practice is both incredibly easy and nearly impossible to prevent. The only reason this man was caught, Yoshikawa said, was because his container of fertilized wagyu eggs must've been "easy" to spot.

These cows are, in many ways, victims of their own popularity. Demand for wagyu beef is on the rise globally, especially in Asian countries like Taiwan, which now buys about one-fifth of Japanese beef exports annually. The meat itself is delicately marbled with fat, rich in omega 3, and low in cholesterol. It's also incredibly delicious and really rare, so much so that it's been called the "Rolls-Royce of beef" and can fetch prices as high as $283 USD per-kilogram on the foreign market.

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That makes it the most-expensive beef out there and a prime candidate for theft. But since it's impossible to smuggle a 750-kilogram live bull through customs, cattle thieves set their sites on a smaller, and more lucrative target—wagyu sperm and eggs. A single straw of frozen sperm from a purebred wagyu bull can go for as much as $4,000 USD, according to US wagyu ranch owners who are among the select few to receive their cows legally from Japan more than four decades ago.

To understand why these cows are so rare, you need to understand a bit of wagyu history. Wagyu just means "Japanese cow," and, in reality, there are four distinct breeds of wagyu in Japan. The reason why these cows are so unique has a lot to do with Japan's centuries-long isolation. The country didn't open completely to foreigners until 1853 and all that time without foreign influence meant that local cows never interbred with foreign breeds.

So later, when they started to experiment with inter-breeding, Japanese ranchers developed a type of cow whose heavily marbled meat is totally unique, and unmistakably tender.

“I’ll never forget my first taste of it,” William Pitchford, of the San Antonio, Texas, Godai Sushi Bar & Restaurant, told the San Antonio Express-News. “Everybody likes to say something ‘melts in your mouth,’ but with this meat, it really does. I compare it a lot to foie gras.”

When diners worldwide developed a taste for the subtle, softer wagyu beef, there were only a handful of cattle outside Japan. In the late 1970s, four bulls were given to the US state of Hawaii as a gift.

“At that time, these were the only Wagyu cattle to leave Japan," Jay Theiler, of Snake River Farms, a US wagyu ranch, toldMaui Time. "The Japanese people consider Wagyu national treasures and it was considered the smuggling of defense secrets to export these cattle."

A small selection of wagyu cattle were sent to the US in 1994 as well, and most of the cows raised outside Japan are either descendants of these original cows or possibly the product of fertilized eggs and sperm smuggled illegally out of Japan.

There are now sizable wagyu ranches in countries like Australia, Brazil, and New Zealand. But there aren't anywhere near enough cows in China to keep up with demand. The country remains closed to Japanese beef imports, which means that the majority of wagyu for sale in China comes from ranches in the US and Australia. The two countries are in talks that could change that any day now, but until then, Japan's ranches remain at risk of cattle theft—of near-microscopic proportion.